Effective control of a low-cost twin-screw food extruder

Abstract

Africa has one of the largest rates of poverty and famine. The hunger crisis in Africa is due to poverty as well as the little availability of nutritional food. Many locally grown crops are available in Africa but are not used to their fullest potential. There is a huge demand for proper food production methods. The food crops in Africa are still cooked in traditional methods by using wood and coal. This method of cooking produces food of very low quality and nutritional value. The reason for this is the over-cooking of food, breaking down proteins and other nutritional components. Extrusion is the process whereby high quality food is manufactured by using simple raw materials. Food extrusion is a process in which a food material is forced to flow, under one or more conditions of simultaneous mixing, heating and shearing, through a die, which is designed to form and/or puff-dry the ingredients. The main disadvantage of extruders available on the market today is the cost per unit. Extruders are currently manufactured in the USA, Europe and the Far East. These machines are very expensive and unaffordable for the African market. The first barrier to overcome is to find a high quality extruder that is affordable for the African economy. This dissertation will provide an introduction to extrusion technology and the application of an effective control system. Chapter 2 contains a literature study of the types of extruders available and discusses the different components of the extrusion process of the co-rotating twin-screw food extruder. The different needs and barriers for developing extrusion technology in developing countries will also be discussed. In Chapter 3 we will discuss the different components that form a control system and the various control modes that can be implemented to improve the control system, reduce offset and enhance stability. Chapter 4 discusses the purpose of each component of the extrusion process, PLC implementation and control-philosophy of each component. Chapter 5 involves a short case study of a corn grit extrusion process. In this chapter we will discuss three basic stages in the extrusion process and the corresponding input parameters for each stage. The relation between extruder screw speed, feeder screw and percentage of water injected into the extruder process, will be investigated. Recommendations and conclusions of this study will be summarised in Chapter 6.